Allow starting sessions from Slack threads with context

[nwparker]

Summary

• allow @mentions inside a Slack thread to start a new session when no session exists for that thread
• seed the new session with prior thread messages as user context
• fetch thread replies with Slack-supported pagination limits

Notes

• Slack app needs the relevant history scopes to read thread replies (e.g. channels:history/groups:history/im:history/mpim:history).

Testing

• npm run build -w @remote-swe-agents/agent-core
• npm run build -w slack-bolt-app